Full Idea
Ernest Nagel defines reduction as the possibility of deriving all laws of one theory by logic and mathematics to another theory, with appropriate 'bridging principles' (either definitions, or empirical laws) connecting the expressions of the two theories.
Gist of Idea
Reduction has been defined as deriving one theory from another by logic and maths
Source
report of Ernest Nagel (The Structure of Science [1961]) by Jaegwon Kim - Philosophy of Mind p.213
Book Reference
Kim,Jaegwon: 'Philosophy of Mind' [Westview 1998], p.213
A Reaction
This has been labelled as 'weak' reduction, where 'strong' reduction would be identity, as when lightning is reduced to electrical discharge. You reduce x by showing that it is y in disguise.